1. Field of the Invention
The present invention relates generally to electronic measuring devices, and more specifically to a system for measuring layer thicknesses using a magnetic probe.
2. Description of the Prior Art
Fabrication of various types of material layers is done in many applications. In some of these applications, it is important to be able to accurately measure the thicknesses of the layers of material which have been applied. In one such application, a layer of a ferromagnetic, virtually nonconductive, material is applied over a relatively thick conductive base layer. A nonferrous, nonconductive top layer, such as paint, may be applied over the ferromagnetic layer. Both the ferromagnetic layer and the top layer may have uneven thicknesses, and it is desirable to be able to measure both of these thicknesses.
Several techniques have previously been developed to measure the thickness of nonconductive layers over ferromagnetic base layers. Measurement of ferromagnetic layer thicknesses over either conductive or nonconductive bases has also been accomplished. These systems make use of eddy current probes to determine layer thickness.
Eddy current probes utilize alternating current through a coil to generate a magnetic field. When placed near a ferromagnetic material, the modification of the magnetic field flux lines by the ferromagnetic material modulates the impedance of the coil. This modulation can be detected, and correlated with the thickness of the material.
With presently existing systems, the thickness of a nonferrous, nonconductive layer over a ferromagnetic base layer can be measured. The ferromagnetic layer contributes to the modulation of the probe's magnetic field, and its contribution must therefore be kept constant. The ferromagnetic base layer must therefore be either thick enough to appear infinitely thick to the magnetic field generated by the eddy current probe, or it must have a constant thickness. Thickness variations in the underlying ferromagnetic base layer will cause erroneous readings in the measurement of the overlying nonferrous, nonconductive layer.
In a similar manner, the thickness of a ferromagnetic layer over a conductive base layer may be measured. Assuming the conductive base layer to be thick enough that it appears infinitely thick to the probe, or constant in thickness, thickness variations in the ferromagnetic layer can be detected by the probe. Measurements in this manner require the probe to either be consistently in contact with the surface of the ferromagnetic layer, or to have a fixed standoff (air gap distance) between the probe and the surface of the ferromagnetic layer. The spacing, if any, between the probe and the ferromagnetic layer affects the modulation of the probe's magnetic field by the ferromagnetic layer, giving erroneous readings if the spacing is not kept constant.
An important disadvantage of these techniques is that they are not capable of simultaneously measuring both the thickness of a ferromagnetic layer over a conductive layer, and the thickness of a nonferrous, nonconductive layer over the ferromagnetic layer. Determinations of layer thickness cannot be made when both the ferromagnetic layer and the overlying layer have an undetermined thickness. Known techniques are also not capable of performing the virtually identical measurement, of the thickness of a ferromagnetic layer over a conductive layer, when the probe tip must be separated from the surface of the ferromagnetic layer by an unknown standoff. This type of situation arises when a ferromagnetic coating, which is still wet due to recent application, is measured using a robot to position the probe. Typically, the robot cannot maintain the probe distance from the surface of the ferromagnetic layer absolutely constant, which negatively impacts the accuracy Of the ferromagnetic layer thickness measurement.
It would be desirable to provide an improved system for measuring the thickness of a ferromagnetic layer over a conductive base layer. It would also be desirable for such a system to accurately measure the thickness of the ferromagnetic layer regardless of the presence of an overlying nonferrous, nonconductive layer, and regardless of a standoff of the probe from the upper surface of the ferromagnetic layer. It would further be desirable for such a system to be capable of determining the thickness of any such overlying layer, or the spacing of the standoff.